Metal treating processes



United States Patent 6 3,355,330 IVLETAL TREATING PROCESSES Josef Rothkegel and Peter Flittner, Frankfurt am Main, and Gerhard Koch, Oberursel, Germany, assignors to Hooker Chemical Corporation, Niagara Falls, N.Y., a 5

corporation of New York No Drawing. Filed Mar. 3, 1964, Ser. No. 349,161 5 Claims. (Cl. 148-615) ABSTRACT OF THE DISCLOSURE This invention relates to improvements in a process for phosphatizing metallic surfaces, and more particularly, it relates to an improved pickling pre-treatment, prior to the application of phosphate coatings to ferrous metal surfaces.

The application of corrosion resistant phosphate coatings to metallic surfaces and in particular, to ferrous metal surfaces, has long been known in the art. In general, such protective coatings are applied by contacting the metallic surface to be protected with an aqueous acid solution of a metal phosphate, such as zinc phosphate, manganese phosphate, iron phosphate, and the like. In order to assure the formation of a good adherent coating on the metallic surfaces, it is important that the surface be substantially free of contamination, e.g. rust, scale, corrosion, and the like, prior to phosphatizing. Accordingly, in the past it has frequently been the practice to give the metallic surfaces a pickling treatment to effect removal of a substantial portion of such surface contamination, thereby insuring a relatively clean surface for the deposition of the phosphate coating.

Insofar as the effective removal of such surface contamination is concerned, the commonly used acid pickling solution containing sulfuric acid or hydrochloric acid have been found to be quite satisfactory and have been used to a great extent. Some difliculties, have been experienced, however, where the acid pickling solutions have not been completely removed from the metal surface prior to phosphatizing. In such instances, as for example with parts having relatively inaccessible places, such as folds or similar overlappings, blind recesses, and the like, the residual pickling solution remaining in these relatively inaccessible places has an adverse affect on the phosphatizing solution, resulting in defective phosphate coatings on the metal. Although these difiiculties are not encountered when using a phosphoric acid pickling solution rather than one of sulfuric acid or hydrochloric acid, there are other disadvantages which make it extremely difiicult to obtain a satisfactory phosphate coating even after using such phosphoric acid pickles.

In using pickling solutions of phosphoric acid, prior to phosphatizing, it has been found that the surfaces of iron or steel treated are attacked by the phosphoric acid solution so that iron is dissolved in the phosphoric acid. As the iron concentration in the solution increases, there is a very rapid decrease of the pickling effect of the solution. Accordingly, unless some measures are taken to maintain the iron content in the pickling solution below about 5 grams per liter, difiiculties are encountered in that the subsequent phosphatizing treatment does not give a consistently satisfactory phosphate coating. In order to overcome these difliculties, it has been proposed in the past that the iron content of the phosphoric acid pickling solution be maintained below about 5 grams per liter by continuously passing these pickling solutions through an ion exchanger. This, however, necessitates added expense for equipment as well as adding an additional step to the process. In view of the advantages to be obtained by the use of a phosphoric acid pickling solution prior to phosphatizing, it would be desirable if the disadvantages encountered with respect to the decrease in pickling action and subsequent poor phosphate coatings which are obtained as the iron content of the phosphoric acid pickling solution increases, could be overcome.

It is, therefore, an object of the present invention to provide an improved method of pickling metal surfaces, and in particular ferrous metal surfaces, prior to phosphatizing, whereby phosphoric acid can be used as the pickling solution.

Another object of the present invention is to provide an improved phosphoric acid pickling composition which may be used to pickle metal surfaces prior to the application of phosphate coatings.

A further object of the present invention is to provide an improved method of imparting corrosion resistance to metal surfaces, and in particular, ferrous metal surfaces, whereby a fine crystalline layer of phosphate material is produced on a metal surface which has been first pickled with a phosphoric acid pickling solution.

These and other objects will become apparent to those skilled in the art from the description of the invention which follows.

Pursuant to the above objects, the present invention includes a process for treating metal surfaces to impart improved corrosion resistance thereto, which process comprises contacting the surface to be treated with a pickling solution comprised of an aqueous solution of phosphoric acid containing arsenic, and, thereafter, applying to the thus-treated surface a corrosion resistant phosphate coating. It has been found that by utilizing as a pickling liquid an aqueous solution of phosphoric acid containing arsenic, iron greatly in excess of 5 grams per liter, can be tolerated in the pickle without incurring difiiculties with respect to obtaining satisfactory, subsequently applied phosphate coating.

More specifically, in the practice of the method of the present invention, an aqueous solution of phosphoric acid containing arsenic, and preferably trivalent arsenic, is used to pickle metal surfaces, and particularly ferrous metal surfaces, prior to phosphatizing such surfaces so as to impart corrosion resistance thereto. Preferably, the phosphoric acid pickling solutions contain the arsenic in amounts within the range of about 10 to about 300 milligrams per liter, with excellent results being obtained when the arsenic is present in an amount within the range of about 50 to about milligrams per liter. Although the arsenic may be incorporated in the phosphoric acid pickling solution in many convenient forms, it is conveniently added as an arsenic compound, such as the oxide, As O the sulfide, AS283, the alkali metal arsenites, e.g. sodium arsenite, potassium arsenite, etc., and the like. Generally, the form in which the arsenic is added to the pickling solution is not critical provided it is soluble in the phosphoric acid and the substituents added, other than arsenic, are not detrimental to the pickling solution, the metal surface being treated on the subsequently applied phosphate coating.

Various phosphoric acid pickling solutions, as are commonly used in the art, may be used in the present method by incorporating therein arsenic in the amounts as indicated hereinabove. Although the aqueous phosphoric acid solutions used are preferably the more dilute solutions, more concentrated phosphoric acid solutions such as those containing as much as 75 percent by weight and more, phosphoricacid may also be used. Typical of the preferred more dilute phosphoric acid solutions which may be 'used are those which may contain from about 2.5 to about 25 percent by weight phosphoric acid and more preferably those containing from about 4 to about 15 percentH PO These pickling solutions, in addition "to the phosphoric acid and the arsenic, may also contain other additives, such as emulsifiers, inhibitors, and the like, as are known to those in the art. Such additives may be present in the pickling solution in amounts up to about 20 percent by weight of the solution, although lesser amounts, e.g., up to about 5-10 percent by weight of the solution are preferably used, typical amountsused being from about 0.01 to about 5 percent. Exemplary of emulsifiers which may be used are the non-ionic surface active agents,

' such as polyglycol ethers of fatty alcohols. Exemplary of the various inhi'bitors'which may be used are thiourea, de-

As has been indicated hereinabove, the addition of these and other emulsifiers and inhibitors, is known to those in the art and although their presence in the pickling solutions may be advantageous in some instances, their use is not essential to the practice of the method of the present invention.

It will be appreciated that various types of corrosion resistant phosphate coatings :may be applied to the ferrous metal surfaces following the pickling treatment as has been described hereinabove. Preferably, these coatings are made up of phosphates of metals such as zinc, iron, manganese and the like. As is known in the art, these coating solutions are generally applied from solutions of acid metal phosphates, e.g., the zinc, iron, or manganese phosphates. It will be appreciated that the quality of such coatings and to a great extent, their ability to resist corrosion, is frequently apparent from visual observation of the coating deposited on the metal surface. Thus, where the coating is an even layer of fine crystalline material,

excellent corrosion protection is generally obtained. If, on the other hand, the coating appears 'tobe somewhat transparent, this is generally evidence that the coating has not been completely. reacted and deposited on the metal su'rface so that, frequently, inferior corrosion protection results. 'It has been found that by use of the pickling'treatment of the present invention, and particularly with coatings containing zinc phosphates, an even layer'of a fine crystalline coating material is consistently obtained.

- It will be appreciated that in carrying out the process of the present invention, the phosphoric acid pickling solution, containing arsenic, may be applied using many of the. conventional techniques. For example, the ferrous metal surfaces to be pickled may be immersed in the pickling solution for a period of-time sufiicient to effect substantially complete removal of rust and other corrosion from thesurface. Alternatively, of course, the pickling solution may be sprayed on the surfaces. to be cleaned.

Frequently, it has been found thatthe consumption of arsenic in the pickling solution is less when using immersion techniques than with other methods sothat this method is often preferred. It is believed that the details of these and other methods for applying the pickling solution to the ferrous metal surface are known to those in the art so thatfurther descriptionof these processes are not necessary. Similarly, standard processing procedures for handling the pickled metal subsequent to phosphatizing as well as during the application of the phosphate coating may also be used. As is typical in such processes, the pickled metal'surface will generally be water rinsed between the pickling and the phosphatizing operation and, if desired,

4 may be treated with an activating solution, such, as, for example, that as disclosed in U.S. Patent 2,874,081.

In order that those skilled in the art may better understand the method of the present invention and the manner in which it may be practiced, the following examples are given. in these examples, unless otherwise indicated, the temperatures given are in degrees centigrade, the times are in minutes, and the amounts are'in grams per liter;

' Example 1 A phosphoric acid picklingsolution is made-containin about grams per liter of H- PO To this pickling solution was added arsenic trioxide (As O in amounts sufficient to provide an arsenic content in the pickling solution of about 0.13 gram per liter. For test purposes, vary ing amounts of iron were also added to thepickling solution. Degreased steel panels were then immersed in the pickling solution fora period of about 10 minutes, the solution being at a temperature of about 50 to 55 degrees centigrade. Thereafter, the panels were removed from the solution, water rinsedand then given a phosphate coating by immersing the panels in a conventional aqueous acid solution of zinc phosphate, containing about 4 to 5 grams per liter of iron, for a period of about 10 minutes, the phosphate solution being at a temperature of about '55, dee grees centigrade. These steel panels were then water rinsed and dried. Using this procedure, the following results were obtained:

Fe content of pickle (.gJl.) Phosphatecoating None Fine crystallineeven layer. 15 a; Do. 30 g Do.

Example 3;

By way of comparison, the procedure of Example 2 was repeated with the exception that the" pickle solution contained no arsenic. Using this procedure, the-following results were obtained: 7 Fe content of pickle (g./l.) rhospha'te coating None Transparent layer.

15' -i. Substantially no n er formation.

The procedures of thc'preceding Examples 1 and '2 are repeated'with pickling solutions containing varying amounts of arsenic Within the range of aboutlllfil to about 0.3 grams per liter, addedas other-arsenic compounds, such as sodium arsenite, andcontaihidg Katherinhibitors, such as hexamethylenete'tr'amin'e and substan tially the same results are obtained.

Fronrthe results obtainedinihe above examples; it is seen that the disadvantageous influence of iron accumulation in phosphoric acid pickling solution is substantially. V a reduced by the incorporation of arsenic in the solution.

This is evidenced by the fine crystalline even phosphate coatings which are produced by phosphatizing theferiou's surfaces after treatment with the phosphoric acid pic-klih'g solution containing arsenic as compared to thesubstantially complete absence of phosphate layers'in the coatings produced after phosphatizing the surfaces following pickling with a phosphoric acid solution having an-ap preciable iron content and containing no arsenic.--

While there have been described variousembodiments of the invention, the methods described are not intended V to be understood as limiting the scope of. the invention as it is realized that changes therewithi n' are pcssibie Phosphate coating and it is further intended that each element recited in any of the following claims is to be understood as referring to all equivalent elements for accomplishing substantially the same results in substantially the same or equivalent manner, it being intended to cover the invention broadly in whatever form its principle may be utilized.

What is claimed is:

1. A process for imparting improved corrosion resistance to ferrous metal surfaces which comprises contacting the ferrous metal surface to be treated with an aqueous phosphoric acid pickling solution containing arsenic in an amount within the range of about to 300 milligrams per liter, maintaining said pickling solution in contact with the metal surface for a period sufficient to effect pickling of the metal surface without substantial formation of phosphate coating on the surface, and, thereafter, contacting the thus-pickled ferrous metal surface with an aqueous phosphatizing solution for a period sutficient to form a corrosion resistant phosphate coating on the surface.

2. The method as claimed in claim 1 wherein the arsenic in the phosphoric acid pickling solution is trivalent arsenic.

3. The process as claimed in claim 2 wherein the trivalent arsenic is present in the phosphoric acid pickling solution in an amount within the range of about to about milligrams per liter.

4. The process as claimed in claim 2 wherein the H PO content of the phosphoric acid pickling solution is within the range of about 25 to about 25 percent by weight of the solution.

5. The process as claimed in claim 4 wherein the H PO content of the pickling solution is within the ran e of about 4 to about 15 percent by weight of the solution.

References Cited UNITED STATES PATENTS 2,314,565 3/1943 Thompson 1486.17 X 2,329,065 9/1943 Lum 1486.15 2,456,947 12/1948 Jernstedt 148-6.15 2,884,351 4/1959 Cavanagh et a1 1486.15 3,082,116 3/1963 Halversen et a1. 11750 RALPH S. KENDALL, Primary Examiner. 

1. A PROCESS FOR IMPARTING IMPROVED CORROSION RESISTANCE TO FERROUS METAL SURFACES WHICH COMPRISES CONTACTING THE FERROUS METAL SURFACE TO BE TREATED WITH AN AQUEOUS PHOSPHORIC ACID PICKING SOLUTION CONTAINING ARSENIC IN AN AMOUNT WITHIN THE RANGE OF ABOUT 10 TO 300 MILLIGRAMS PER LITER, MAINTAING SAID PICKLING SOLUTION IN CONTACT WITH THE METAL SURFACE FOR A PERIOD SUFFICIENT TO EFFECT PICKLING OF THE METAL SURFACE WITHOUT SUBSTANTIAL FORMATION OF PHOSPHATE COATING ON THE SURFACE, AND, THEREAFTER, CONTACTING THE THUS-PICKLED FERROUS METAL SURFACE WITH AN AQUEOUS PHOSPHATIZING SOLUTION FOR A PERIOD SUFFICIENT TO FORM A CORROSION RESISTANT PHOSPHATE COATING ON THE SURFACE. 